all: define Berlin hard fork spec

eth/ethconfig/config.go

@@ -198,6 +198,9 @@ type Config struct {
 
 	// CheckpointOracle is the configuration for checkpoint oracle.
 	CheckpointOracle *params.CheckpointOracleConfig `toml:",omitempty"`
+
+	// Berlin block override (TODO: remove after the fork)
+	OverrideBerlin *big.Int `toml:",omitempty"`
 }
 
 // CreateConsensusEngine creates a consensus engine for the given chain configuration.